1. Field of the Invention
The present invention relates to a method for obtaining data in phase-contrast MR angiography which is sensitive to flow (movement) in three orthogonal directions. In particular, it relates to a method of reducing acquisition time and increasing sensitivity in phase-contrast MR angiography.
2. Description of Prior Art
Two basic physical principles can be used to acquire flow images with magnetic resonance (MR): time of flight and phase change effects. The first relies on signals from unsaturated flowing nuclear spins relative to saturated stationary background tissue.
Phase effects are based on the fact that spins moving along magnetic field gradients acquire a phase shift of their transverse magnetization that is directly related to the velocity of the flowing spins. Principles of this technique are described in an article entitled "A Flow Velocity Zeugmatrographic Interlace for NMR Imaging in Humans" by Paul A. Moran, Magnetic Resonance Imaging, Vol. 1, pp. 197 to 203, 1982.
A pulse sequence for producing an angiogram based on the net change in phase of moving spins was suggested by C. L. Dumoulin et al in Radiology 1986, 161 (pp. 717 to 720). In this sequence a flow-encoding gradient pulse is applied to a set of spins and then another pulse is applied some time later with reversed polarity. A spin that has not changed position along the axis of the flow-encoding gradient will experience no net change in phase. A spin that has moved along the axis of the gradient, however, will have a net change in phase. This change in phase can be measured by, for example, repeating the data acquiring pulse sequence without flow encoding gradient pulses or with inverted flow encoding gradient pulses. Taking the difference of the two sets of acquired data will result in cancellation of the signals from non-moving spins, since their signals are identical. Signals from moving spins, however, will have different phases in the two data sets and thus will be detected. The magnitude of the difference between the two sets of acquired data can be displayed as an angiogram wherein signal intensity is a function of blood flow in the flow sensitive direction. This known method is referred to as the phase-contrast method. However, a long acquisition time is necessary since two separate data sets must be acquired for each flow direction. If sensitivity in all three orthogonal directions is desired, it is required to take six measurements to produce the complete angiogram.
Similar methods are described by C. L. Dumoulin et al in an article entitled "Three-Dimensional Phase Contrast Angiography" in Magnetic Resonance in Medicine, 9, pp. 139 to 149, 1989.
According to a method described by R. Hausmann et al in the article "Reduction of Scan Time with a Modified Phase Contrast Acquisition Scheme" in Book of Abstracts, Vol. 1, page 402, Society of Magnetic Resonance in Medicine, Aug. 18 to 24, 1990, four sequences are performed in an interleaved manner. Three of the sequences are phase-sensitive to flow in three orthogonal directions and the forth sequence is flow compensated. A complex data set is obtained during each sequence. By taking the complex difference of each of the flow encoded data sets and the one flow compensated raw data set, signal from stationary tissue is cancelled. The subtracted data sets are Fourier-transformed, leading to three sets of flow magnitude images. These three data sets are then combined in a voxel-by-voxel method to obtain a 3D data set which is sensitive to flow in all three directions. This method provides short acquisition times. However, the signal-to-noise ratio of the acquired signals along each direction are not as good as those acquired by sequences which are simultaneously sensitive to spins in two directions, e.g., x,y and x,z.
A further method for obtaining signals for phase-contrast MR angiography is described by N. J. Pelc et al in Book of Abstracts, Vol. 1, page 475, Society of Magnetic Resonance in Medicine, Aug. 18 to 24, 1990. It also applies four sequences: the first sequence is a reference sequence, the second sequence encodes the spins in the x and y directions, the third in the x and z directions and the fourth in the y and z directions. Thus, these sequences are simultaneously sensitive to flow in two orthogonal directions.
It is an object of the present invention to provide a method for obtaining data in phase-contrast MR angiography which, with a short acquisition time, is sensitive to flow in three orthogonal directions.
It is a further object of the invention to provide a phase-contrast MR angiography method with high sensitivity to phase differences caused by flow of the nuclear spins.
It is still a further object of the invention to provide a phase-contrast MR angiography method which is insensitive to patient motion during measurement.